Selector magnet driver



Feb. 15, 1966 c. R. WINSTON SELECTOR MAGNET DRIVER Filed June 22, 1962 mm wP S I o 4 1| 2 )2? 4 5 4 4 7 E a -||l||ll.. I T mr TEL 0 cN 60 5 LM E o s 3 w 4 3 8 2 m a m INVENTOR CHARLES R. WINSTON ATTORNEY United States Patent 3,235,775 SELECTOR MAGNET DRIVER Charles R. Winston, Deerfield, Ill., assignor to Teletype Corporation, Skokie, 11]., a corporation of Delaware Filed June 22, 1962, Ser. No. 204,569 16 Claims. (Cl. 317143.5)

This invention relates to a system for energizing an electromagnet coil and more particularly to a low noise driving circuit for a selector magnet employed in teletypewriter machines which operate in response to signals having substantially rectangular current wave form characteristics.

The selector magnet of teletypewriter machines comprises an iron core inductor or coil and an armature which is operable thereby. Upon application of energizing current pulses through the inductor, having sutlicient magnitude to attract the armature, a conversion from electrical to mechanical energy is effected. The mechanical energy is then utilized to store intelligence, for example, by printing a character or by punching holes representing the intelligence in paper tape.

The current pulses are normally receivable in groups of five or more information pulses preceded by a start pulse and terminated by a stop pulse. These pulses or bits, seven or more, occur seriatim. The information pulses will assume one or the other of two current conditions representative of the information they represent; the start pulse always has one of the two current conditions and the stop pulse always has the other current condition. The current condition of the stop pulse is such that when it is applied to the selector magnet coil, it maintains that coil in an energized condition; and this condition continues until a start pulse is received by the selector magnet coil. The start pulse deenergizes the coil, thus initiating operation of the teletypewriter machine by releasing the armature of the selector magnet; sothat upon reception of the five or more information pulses, the selector magnet operates associated selector mechanisms thereby to allow the teletypewriter machine to print or punch the character represented by the information pulses. This type of operation is more fully described in United States Patent No. 2,339,313, issued to W. J. Zenner on Jan. 18, 1944.

The normal method utilized for periodically energizing the selector magnet coil of teletypewriter machines is to switch the current through the coil on and 01f thereby to cause energization and deenergization of the coil. The simplest approach to this type of operation is to connect a switch in series with the current supply to the coil. Such a switch may be opened and closed by operating a relay in response to the signal pulses on the line, or the switch may be an electron tube or transistor which is turned on and otf in response to the signal pulses on the line. For normal telegraph applications the above-mentioned types of systems for energizing and deenergizing the selector magnet coil give satisfactory results. However, when systems utilizing the general on/otf principle of operation are employed, a relatively large amount of RF noise is generated which may be conducted out onto the line and/ or radiated from the system.

It has been found that when a transistor saturates or when it is fully cut off, the transistor generates RF noise which may be conducted or radiated to other portions of the circuit. Since the normal operation of transistorized selector magnet driver circuits drives the transistors into saturation and cuts them off in response to mark and space signals, respectively, such systems inherently are noisy. It also is known that the conventional telegraph signal which is comprised of a series of square wave pulses each having a steep wave front or a fast rise time causes the generation of RF noise whenever it is utilized to operate driving circuits such as referred to above.

Some present telegraph receivers are operated in environments in which such RF noise cannot be tolerated. It is necessary, therefore, to employ a driving circuit for the selector magnet coil which does not possess the previous inherent disadvantage of generating RF noise.

Another disadvantage which is inherent in many of the selector magnet driving circuits of the prior art is that these circuits are voltage responsive systems, that is, they depend upon a voltage drop across the input terminals of the selector magnet driver to initiate operation of the driver circuit. Since the telegraph systems in common use today are essentially constant current systems rather than constant voltage systems, it is apparent that if a large number of such prior art voltage responsive driver circuits are to be connected in series on a telegraph signal line, the line battery or supply voltage must be increased accordingly for each added driver circuit which is used. If a limited line battery voltage is available, only a limited number of such voltage responsive systems may be connected in series on the line in order that the con stant current aspect of operation may be maintained. Accordingly, it is highly desirable to have the selector magnet driver circuit present a low impedance to the line signal so that a large number of driver circuits may be connected in series on the line without the necessity of using a high, line supply voltage.

Accordingly, it is a primary object of this invention to provide a selector magnet circuit which does not generate RF noise above .07 microvolt in the frequency range from 14 kilocycles to megacycles.

It is another object of this invention to use an electronic circuit for energizing a selector magnet coil in which the active circuit components are never turned fully oil nor fully on.

It is another object of this invention to provide a transistor driving circuit for an electromagnet coil in which the transistors are never driven to saturation and are never driven to cutoff.

It is another object of this invention to provide an electromagnet driver circuit which presents little or no impedance to the signal line.

It is a more specific object of this invention to connect diodes, having exponential voltage-current characteristics, in the emitter and collector circuits of transistors, for preventing the transistors from saturating or from cutting oil.

It is a further object of this invention to provide a selector magnet driver circuit which is a current responsive circuit.

In a preferred embodiment of this invention, a pair of input transistors are connected to operate as a class A circuit in which current conduction through the second transistor ranges from a low value near zero for space to a predetermined high value for mark. When there is no current present on the line, which corresponds to an open line or space, a small amount of current is conducted through the selector magnet coil and the collector-emitter path of the second transistor from the local DC. power supply. This low current causes a predetermined voltage to be established at the base of the first transistor, the collector of which is connected to the base of the second transistor to bias the second transistor to conduction for maintaining this small amount of current flow therethrough. The current flow through the coil is insufiicient to energize the coil for eliecting operation of the selector magnet.

When a mark is present on the line, it is represented by current flow on the line; and the current, instead of being shunted through an external resistance, flows from a first line terminal through the internal DC. power supply, the selector magnet coil, and the second transistor back to the other line terminal. This line current tends to bias the first transistor to a different state of conduction. As a result, the potential on its collector changes and biases the second transistor to conduct more heavily in order to maintain the voltage difference between the base and emitter of the first transistor constant. The increased current conducted by the second transistor flows through the selector magnet coil to effect energization of the coil thereby causing the selector magnet to operate. A string of diodes are connected to conduct current from the DC. power supply to the collector of the second transistor and prevent-the transistor from saturating by establishing a predetermined voltage on the collector when it is conducting high current for'mark.

During the initial phase of energization of the coil, before the current flowing through it reaches a quiescent or steady state value, a' low impedance path is connected between the DC. power source and the inductor coil to allow a rapid energization or build up of current through the coil. When the current reaches a predetermined level or magnitude it substantially opens the low impedance currentpath, and. current supplied to the inductor then flows through a higher impedance path which limits the current to a safe predetermined level which may be different from the level of current which opens the low impedance path. I

The low impedance path for this initial surge of current is provided by the collector-emitter path of a control transistor. The high impedance path consists of a current limiting resistor which is connected in parallel with the collector-emitter path of the control transistor. When the current is below the above-mentioned predetermined value, the control transistor is rendered conductive and current flowing to the inductor passes through the transistor. A plurality of diodes are connected in series in the collector circuit of the transistor and are poled to conduct in their forward direction. These diodes operate to prevent the transistor from saturating during the period of its conduction by maintaining a predetermined voltage on the collector. When the current isabove the afore-' mentioned predetermined value, the current flow through the transistor is reduced to near zero presenting efiectively an open circuit to current flow therethrough, and essentially the only available current path is then through the higher impedance current limiting resistor. diodes are connected in series in the emitter circuit of the transistor to maintain the voltage on the emitter just sufiiciently positive with respect to that on the base toallow this small amount of current flow Further objectsand features of this invention will become apparent to those skilled in the .art upon consideration of the following detailed specification taken in conjunction with the drawing in which:

FIG. 1 shows a circuit diagram of a preferred power supply to be used with this invention;

FIG. 2 shows a circuit diagram of a preferred embodiment of the invention; and

FIG. 3 shows wave forms useful in understanding the operation of the circuit shown in FIG. 2.

In FIG. 1 of the drawing there is shown a preferred embodiment of a low noise power supply particularly adapted for use with the low noise selector magnet driver circuit of this invention. Generally, the power supply 10 is a full wave rectifier comprising a pair of semiconductor rectifier diodes 11 and 12 which are connected to opposite ends of a center tapped secondary winding 13 of a transformer 14, the primary winding of which is supplied with power from a suitable A.C. source.

In order to prevent any RF radiation or conduction of RF noise from taking place in the operation of the power supply 10, a pair of current limiting resistors 15 and 16 and a pair of inductors 17 and 18 are connected A string of in series with the diodes 11 and 12, respectively. The

resistors 15 and 16 limit the peak current flowing throughthe rectifiers and the inductors 17 and 18 eliminate transient voltage spikes which would otherwise appear. It is necessary to use the inductors 17 and 18 because semiconductor diodes presently available do not switch off immediately when an inverse potential is applied across them but pass a sharp transient spike of voltage in the inverse direction immediately after the inverse voltage from the source is applied across them. The inductors 17 and 18 eliminate these transient voltage spikes and prevent them from causing any RF interference in the circuit.

Conventional semiconductor diodes suitable for use as rectifiers 11 and 12 also exhibit a tendency to oscillate slightly when they are switched into and out of conduction, and the inductors 17 and 18 also serve to prevent this oscillation from occurring. It is to be noted that in ordinary applications this tendency for slight oscillation or ringing presents no problem; but in extremely low noise applications such as in the low noise selector magnet driver circuit of FIG. 2 with which the power supply 10 is used, such oscillations might cause intolerable RF interference if they were not eliminated. The RF filter of which resistors '15 and 16 and inductors 17 and 18 are a part, is further improved by the addition of four capacitors 19, two of which are connected between the center tap terminal of the secondary winding 13 and the junctions of the resistors 15 and 16 with the'ends of the secondary winding 13. The other two capacitors 19 are connected between the center tapped terminal of the transformer winding 13 and the junctions of the resistors 15 and 16 with the inductors Hand 18.

A conventional ripple filter 20 is connected across the output terminals of the rectifier circuit, and the output voltage is regulated by means of a Zener diode 21 connected in series'withan inductor 22 and a plurality of diodes 23 between the output terminals of the rectifier.

It-has been found that a relatively inexpensive Zener diode 21 having relatively wide tolerance may be used in this circuit if a proper number of trimming diodes 23 are connected in series with it. These diodes are connected to conduct current in their forward direction across the output terminals of the rectifier and each exhibits a fixed voltage drop thereacross over a wide range of currents; By using a plurality of these trimimn-g diodes 23, the percentage of tolerance in the circuit is improved, and the overall regulation attained by the series combination of the-diodes 23 and the Zener diode 21 is equivalent to that which would be attained by using an expensive Zener diode having close tolerances. The number of diodes 23 necessary in the circuit varies depending upon the characteristics of the particular Zener diode used in each circuit.

' The inductor 22 in conjunction with a pair of capacitors 24 and 24a acts as afilter to eliminate RF noise generated in the Zener, diode 21 from the output of the pow-er supply 10. In conventional filters the inductor of the filter is connected in series with the output terminal;

thus, the conventional location for the inductor 22 wouldv that a space is present on the signal'line. This would be represented by no current on the line or the equivalent of an open switch between the input terminals 25 and 26. When this space condition occurs, a pair of NPN input 3 transistors 27 and 28 are in a first quiescent state of conduction' in which the transistor 28 conducts a small amount of current from the power supply through a variable resistor 38, a current responsive system 28 (the operation of which will be described hereinafter) and a selector magnet coil 30 to the other side of the power supply. This current is of insuflicient magnitude to cause energization of the selector magnet coil 30. The voltage on the emitter of the transistor 28 is positive with respect to the negative side of the power supply It) and this positive potential is applied to the base of the transistor 27 through a resistor 31. Since the emitter of the transistor 27 is connected directly to the negative side of the power supply 10, the transistor 27 is therefore biased into conduction and it conducts current from the positive side of the power supply 10 through a resistor 32. Due to the regenerative coupling of the transistors 27 and 28 only enough current is conducted through the transistor 27 to maintain the bias voltage on the base of the transistor 28 just suflicient to draw this small amount of current through the transistor 28. The parameters of the circuit are chosen to give the desired amount of current conduction through the transistor 28.

It is to be noted that both transistors 27 and 28 conduct current during this quiescent or space condition of the circuit. This type of operation is in opposition to the normal on/otf type of circuit commonly used for systems of this general type since the on/ off approach would be to connect the transistors 27 and 28 in a bistable circuit configuration wherein the transistor 28 would be fully oif for this space condition.

When a mark is present on the signal line, current is allowed to flow through the line by operation of a line keyer which is equivalent to the closing of a switch in series with a line battery connected across the input terminals and 26. The line keyer may be of any suitable type, but is preferably of the type disclosed in the copending application to C. R. Winston, Ser. No. 204,356, filed June 22, 1962.

Although the line keyer forms no part of this invention, the line battery and the switch representative of the line keyer have been shown in dotted lines connected to the input terminals 25 and 26 in order to facilitate an understanding of the operation of this system. When the switch or line keyer is closed, current representing a mark begins to flow on the line between input terminals 25 and 26. Immediately upon closing the keyer switch the negative and positive potentials of the line battery are applied to the base and the emitter, respectively, of the transistor 27 and tend to turn the transistor 27 off. This tends to reduce the current flowing through the transistor causing a rise in potential at the collector of the transistor 27 which is applied to the base of the transistor 28 thereby causing the transistor 28 to conduct more current.

The current path for the line current is from the positive side of the line battery applied to the terminal 26, through a diode 33, the power supply 10, the current responsive circuit 29, the selector magnet coil 30, the transistor 28 and the resistor 31 to the negative side of the line battery connected to the terminal 25. The transistor 28 conducts increased current in order to maintain the voltage difference between the base and the emitter of the transistor 27 the same as it is during the space or quiescent condition of the circuit. However, in order to maintain this difference during marking intervals, the current flow through the transistor 28 is now equal to or greater than the line current on the telegraph signal line.

In order to obtain higher currents than the line current through a portion of the circuit, a resistor 34 and a pair of diodes 35 are connected in series between the negative side of the power supply 10 and the junction of the emitter of the transistor 28 with the resistor 31. The ratio between the resistance of the resistor 31 and that of the resistor 34 is chosen sothat with the desired line current flowing through the resistor 31, sufficient additional current flows through the resistor 34 and diodes 35 to cause approximately 90 milliamperes of current to flow through the transistor 28 during the mark interval. For example, if the line current is 60 milliamperes, milliamperes of current flows through the resistor 34 and diodes while milliamperes of current flows through the resistor 31, resulting in a total of 90 milliamperes of current being drawn through the transistor 28.

Between 60 and milliamperes of current are generally required to flow through the selector magnet coil 30 to energize it for effecting operation of the selector magnet during the mark interval. Any additional current above this amount which is drawn by the transistor 28 is supplied from the positive side of the power supply 10 through a first string of series connected diodes 36 and a second string of series connect-ed diodes 37 which maintain a constant voltage drop thereacross regardless of the amount of current which is drawn through them. Thus, if increased current flows through the transistor 28, the voltage drop across the selector magnet coil 30 and the current responsive circuit 29 tends to rise. However, this voltage drop cannot exceed a predetermined value set by the diodes 36 and 37, and the additional current is drawn through the diodes 36 and 37.

Thus, the voltage on the collector of the transistor 28 is maintained constant. This voltage is chosen to be of such a value that the transistor 28 cannot saturate regardless of the amount of current which is being drawn through it. This is accomplished by choosing this voltage to be such that the collector voltage of the transistor 28 cannot enter the saturation region. If the collector voltage were not clamped to this predetermined value by the action of the diodes 36 and 37, sufficient bias is applied to the base of the transistor 28 to otherwise drive the transistor into saturation during marking intervals.

In order to more fully understand the manner in which the strings of diodes 36 and 37 achieve the foregoing clamping action to prevent saturation of the transistor 28, reference is made to FIG. 3 which shows a typical characteristic curve X which shows the voltage drop across the diodes plotted against the current flowing through them. The curve X is a nonlinear curve exhibiting the simple exponential voltage-current characteristic of the diodes. For increasing currents between 0 and a value A, the voltage drop across the diodes rises rapidly, but remains relatively constant for any additional increase in current between the value A and a second value B. The simple exponential curve X is contrasted with a curve Y which is a typical voltage-current curve for a linear resistor.

The voltage at which the plateau or relatively flat portion of the curve X occurs is established by the individual voltage drop across each diode and may be increased or decreased by correspondingly increasing and decreasing the number of diodes connected in series. For example, if the voltage drop for the applied currents between the values A and B for each diode is .5 volt and a total voltage drop of 10 volts is desired, it is necessary merely to connect twenty such diodes in series and the total drop across them is the desired 10 volts.

It should be noted that the aforementioned currentvoltage characteristics of the diodes are those which are achieved by operating the diodes in their forward current conducting direction. Since the diodes are operated in their forward current conducting direction, it is not necessary for them to exhibit good rectifying characteristics or blocking characteristics in the inverse direction; and consequently they may be of an inexpensive type, with the result that even the use of a large number of them in a string to achieve the desired voltage drop does not render the cost of the string prohibitive. The constant voltage characteristic of the diodes is utilized by operating the strings of diodes in the current regions between A and B.

These strings of diodes have been used in place of Zener diodes for one primary reason, namely, that when the current is increased from O to point A, the diode conducts throughout this region and the voltage drop thereacross rises exponentially to the value it has at the point A. There are no sharp transitions or changes of conduction to cause the generation of RF noise since the voltage current curve has only gradual transitions or rounded wave form characteristics. A Zener diode, on the other hand, exhibits a sudden avalanche of current flow therethrough once the breakdown voltage across the diodes is exceeded. This transition from nonconducting to conduction is sharp and generates RF noise which is intoler' able in the low noise selector magnet driver system of this invention.

As mentioned previously, the diodes 36 and 37 are strings of series connected diodes, the number of which is determined by the voltage drop desired across them. In order to distinguish these and other strings of diodes from conventional single diodes on the drawing, a circle is drawn around a conventional diode symbol to represent a string of diodes whereas conventional single diodes are shown in the drawing without circles.

During the space condition, when the transistor 28 is drawing only a small amount of current through the selector mag-net coil 30, the current flow to the coil 3t) from the power supply 10 passes through a variable resistor 38, a string of diodes 39 and the collector-emitter path of a control transistor 40. A predetermined bias potential is applied to the base of the transistor 40 by means of a voltage divider comprising the string of diodes 36, a resistor 41, and a string of diodes 42. The voltage drop across the string of diodes 36 is constant since the current flowing through it is between the values A and B shown in FIG. 3. This voltage drop causes the base of the transistor 40 to be biased negatively with respect to its emitter since the voltage drop across the resistor 38 and the diodes 39 due to the current flowing through them is not as great as the drop across the diodes 36. A current limiting resistor 43 and a string of diodes 44 are connected in series between the collector of the transistor 40 and the junction of the resistor 38 with the string of diodes 39. The transistor 40 acts as a shunt or short circuit connected across the resistor 43 and diodes 44 and little or no current flows through the resistor 43 and the diodes 44.

However, when the line keyer is closed, changing the signal on the line from spacing to marking, the action of the transistors 27 and 28, as explained previously, causes the transistor 28 to conduct heavily. The control transistor 40 acts as a low impedance path connected in serieswith the selector magnet coil 30 thereby causing practically the full available potential to be applied across the selector magnet coil 30. This enables a rapid build up of current through the selector magnet coil 30 to take place.

A string of diodes 45 is connected between the collector of the transistor and the string of diodes 42. These strings of diodes 42 and operate in a manner similar to the operation of the strings of diodes 36 and 37 to prevent the transistor 40 from saturating when it is in its high conductive state by clamping the collector of the transistor 40 to a predetermined voltage which is outside the saturation region of the transistor. This is accomplished since the voltage drop across the diodes 45 and 42 is constant regardless of the amount of current flowing through them within the operating range of the system.

As the current through the resistor 38 and transistor 40 increases, the voltage drop across the resistor 38 increases correspondingly until it reaches a level at which the combined voltage drop across the resistor 38 and the diodes 39 applied to the emitter of the transistor 4% approaches the potential at its base thereby rendering the transistor 40 almost nonconductive. The string of diodes 39 is then operated at or below point A but above Zero on its characteristic curve X as shown in FIG. 3 and prevents the transistor from becoming fully cut off by allow-' ing a small amount of current to flow therethrough.

When the transistor 40 is in this low conductive state, it presents a higher impedance flow to current flowing therethrough than the impedance of the current limiting resistor 43. As a result, the energization current for the selector magnet coil 30 now flows from the positive side of the power supply 10 through the resistor 38, the current limiting resistor 43 and string of diodes 44 to the selector magnet coil 30. The resistor 43 acts as a current limiting resistor and has a much higher impedance than the emitter-collector path of the transistor 40 when the transistor 40 is in its high conductive state. As a con sequence the energizing current flowing through the selector magnet coil 30 is limited to a safe predetermined operating level of the selector magnet coil 30 by the resistor 43. The string of diodes 44 are used to improve the switching transition from high conduction to low conduction of the transistor 40.

As stated previously any current in excess of the desired operating current drawn through the selector magnet coil 30 by the transistor 28 is supplied to the transistor 28 through the strings of diodes 36 and 37.

When the line signalagain returns from a marking to a spacing condition, the transistor 28 assumes its first quiescent conducting state in which only a small amount of current flows therethrough, thereby effectively opening the circuit between the selector magnet coil 30 and the negative side of the power supply 10. A string of diodes 46 is connected between the selector magnet coil 30 and the positive terminal of the power supply 10 to clamp the coil to the potential of the positive power supply 10 when the energizing current fiow therethrough is terminated. A plurality of diodes is used in order to limit the peak voltage across transistor 28 while the energy in the coil 30 is dissipated, and these diodes also exhibit the voltage current characteristics shown in curve X in FIG. 3.

In order to provide a fuller understanding of the operation of the system, the following illustration is given, showing the various potentials which occur in the circuit with reference to some specific values. As an illustration, assume that the potential of the power supply 10 is 48 volts and that the potential of the line battery is also 48 volts. When the circuit is in its quiescent or spacing state, the emitter of the transistor 27 is connected to the negative side of the power supply 10 which for the purposes of illustration is considered to be zero volts. In order to maintain the desired quiescent current flow through the transistor 28, the voltage on the base of the transistor 27 must be .5 volt positive with respect to the potential on its emitter. The current flow through the transistor 28 and the resistor 31 to the base of the transis tor 27 is just sufiicient to maintain this potential difference for the quiescent condition.

Now assume that the line keyer or switch on the line is closed representing a mark condition. Current then flows through the diode 33 from the positive side of the 48 volt line potential. The voltage drop across the diode 33 is chosen to be just equal to the voltage drop across the base emitter path of the transistor 27. Consequently, the potential drop across the diode 33 is .5 of a volt. The voltage-current characteristics of the diode 33 in its forward current conducting direction is shown in curve X of FIG. 3. The current flow through the diode 33 is chosen so that it is somewhere Within the range between the points A and B as shown in FIG. 3. Thus the voltage drop across the diode will be the same regardless of the current flow through it. Since this voltage drop is chosen to be .5 of a volt, the potential now present on the emitter of the transistor 27 is 47.5 volts.

The line battery and the power supply 10 are connected in series and poled in the same direction, which causes the potential on the positive side of the power supply 10 now to be 95.5 volts. The current flow from the positive side of the power supply 10 takes place through the resistor 38 the voltage responsive circuit 29, the

selector magnet coil 30, transistor 28 and resistor 31 to the negative side of the line battery 48. As stated previously, the transistor 28 conducts sulficient current to cause the potential on the base of the transistor 27 to be .5 volt positive with respect to the potential on its emitter. Any tendency for the transistor 28 to conduct more current tends to cause the transistor 27 to become more conductive. The transistor 27 then tends to draw 'more current through the resistor 32 causing a more negative bias voltage to be applied to the base of the transistor 28 thereby rendering it less conductive. Thus any tendency of the transistor 28 to conduct any more than this desired amount of current is counteracted.

As a result of the foregoing, when the potential on the base of the transistor 27 reaches plus 48 volts, the circuit is in a steady marking condition and will remain so as long as the line keyer circuit is closed. It is to be noted that the plus 48 volts applied to the base of the transistor 27 during the marking interval is also present on the negative input terminal 25. Thus, plus 48 volts is present on both the input terminals and 26 when the desired current flow for mark is present on the line.

The significance of this operation is that the selector magnet driver circuit of this invention presents zero impedance to the line circuit. As a consequence, any desired number of selector magnet drivers of this type may be connected in series on the line Without causing any voltage drop to occur on the line. The action of the diode 33 causes this zero impedance condition to occur. If the diode 33 were eliminated from the circuit, it readily apparent that the system would present a negative impedance to the line; and if the resistance of the diode 33 were greater than the base-emitter resistance of the transistor 27, the circuit would present a positive impedance to the line. However, by matching the impedance of the diode 33 to the impedance of the base-emitter of the transistor 27, the afore-mentioned zero impedance of the circuit to the line is obtained.

In order to protect the system from surges or hits due to lightning or from accidental reversal of polarity of the signal applied to the terminals 25 and 26, a click suppressor or two-way conducting diode 4'7 is connected across the input terminals. The click suppressor 47 consists of a PN semiconductor junction and an NP semiconductor junction connected in parallel. Since the operation of click suppressors to perform this type of protection is well known no further discussion of the click suppressor 47 will be presented here. Capacitors 48 and 49 interconnect the transistors 27 and 28 to prevent parasitic oscillation from taking place in the circuit of which these transistors are a part. Capacitors 50 and 51 act to filter out any RF noise which might possibly be generated by the transistors in the operation of the circuit. Since the transistors are neither driven to saturation nor to cut ofi, little or no such RF noise is actually generated by them.

The low noise selector magnet driver of this invention is capable of operating without generating any RF noise having magnitude of over .07 microvolt in the frequency range from 14 kilocycles up to 100 megacycles. The system also presents a zero impedance to the telegraph line with the result that any desired number of select-or magnet driver systems built according to this invention may be connected in series on the line Without necessitating an increase in the line battery voltage in order to maintain a constant line current.

Although the system which has been described and illustrated herein is a prefrred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made in this embodiment without departing from the spirit and scope of the invention.

What is claimed is:

1. A circuit for energizing the coil of an electromagnet in response to an external signal including (a) a first transistor having at least base and emitter electrodes,

(b) a second transistor for conducting current through said coil from a current source,

(c) means for applying said signal across the base and emitter electrodes of said first transistor,

(d) means interconnecting said first and second transistors for causing current to flow through said second transistor and said coil to maintain a predetermined potential difierence between the base and emitter electrodes of said first transistor,

(e) said external signal being applied in series aiding relationship with the current source to cause said current through said second transistor to be of one value when no external signal is applied to said first transistor and to be of a higher value when said external signal is applied to said first transistor, and

(f) means for preventing said second transistor from saturating.

2. A circuit for energizing the coil of an electromagnet in response to external signals including (a) first and second transistors each having base, emitter and collector electrodes,

(b) means connecting said coil between a current source and the collector of said second transistor,

(c) first means for connecting the emitter of said second transistor to the base of said first transistor,

(d) second means for connecting the collector of said first transistor to the base of said second transistor,

(e) said first and second connecting means causing current to flow through said second transistor and said coil to maintain a predetermined potential difference between the base and emitter electrodes of said first transistor, said current flowing through said coil being insufiicient to operate said electromagnet when no external signals are applied, and

(f) means for applying said external signals across the base and emitter electrodes of said first transistor,

(g) said external signals being applied in series aiding relationship with the current source to cause increased current to flow through said second transistor and said coil, said increased current being of sufiicient magnitude to operate said electromagnet.

3. A circuit for energizing the coil of an electromagnet in response to an external signal including (a) first and second transistors having base, collector and emitter electrodes,

(b) means for connecting said coil between the collector of said second transistor and one side of a current source,

(c) means for applying said external signals across the base and emitter electrodes of said first transistor,

(d) first means for connecting the emitter of said second transistor to the base of said first transistor,

(e) second means for connecting the collector of said first transistor to the base of said second transistor, and

(f) means connecting the emitter of said first transistor with the other side of the current source,

(g) said first and second connecting means causing current of one value to flow through the collectoremitter path of said second transistor and said coil when no external signals are applied to said first transistor, said current being of insufiicient magnitude to operate said electromagnet,

(h) said external signals being applied in series aiding relationship with the current source to cause increased current to flow through said collector-emitter path of said second transistor and said coil, said increased current being of sufiicient magnitude to operate said electromagnet.

4. A driving circuit for energizing an electromagnet coil in response to signals on an external line including (a) a first transistor having at least base and emitter electrodes,

(b) current responsive means connected between said coil and a current source, said current responsive means presenting a low impedance to current flowing through said coil when said current is low and presenting an increasingly higher impedance to said current as said current is increased,

(c) a second transistor for conducting current through said coil from said current responsive means, and

(d) means interconnecting said first and second transistors for causing current to flow through said sec ond transistor and said coil to maintain a predetermined potential difference between the base and emitter electrodes of said first transistor,

(e) means for applying said external signals across the base and emitter electrodes of said first transistor,

(f) said external signals being applied in series aiding relationship with the current source to cause the current flowing through the second transistor to be of one value in the absence of signals on said external line and to be of another higher value when signals are applied on said external line.

5. A driving circuit for energizing an electromagnet coil in response to signals on an external line including (a) first and second transistors having base, collector and emitter electrodes,

(b) a control transistor and a current limiting resistance connected in parallel,

I (c) current responsive means connected between a current source and said parallel-connected control transistor and resistor for controlling the conduction of said control transistor in response to the current flowing through said coil, said current responsive means causing said control transistor to become less conductive as said current through said coil increases,

(d) means for connecting said coil between the collector emitter path of said second transistor and said parallel connected control transistor and resistor,

(e) means for applying said external signals across the base and emitter electrodes of said first transistor, and

(f) means for interconnecting said first and second transistors in a regenerative feedback circuit configuration for causing current of one value to flow through the collector-emitter path of said second transistor and said coil when no external signals are applied to said first transistor, said current being of insuflicient magnitude to energize said coil,

(g) said signals on said external line being applied in series aiding relationship with the current source for causing increased current to flow through said collector-emitter path of said second transistor and said coil, said increased current being of sufficient magnitude to energize said coil.

6. Apparatus according to claim having means for preventing said second transistor and said control transistor from saturating.

7. Apparatus according to claim 6 wherein said means for preventing said second transistor and said control transistor from saturating comprise a plurality of diodes'connected in series between the collectors of said transistors and the power supply.

8. In an energizing circuit for an electromagnet coil,

(a) a control transistor, the collector-emitter path of i which is included in a path for conducting current from a source to said coil,

- (b) a current limiting resistor connected across said current conducting path,

(0) means responsive to the current flowing through said coil for decreasing the conductivity of said transistor as said current through said coil increases 12 thereby causing increased current to flow through said current limiting resistor, and

((1) means for preventing said transistor from being driven to cutoff.

9. In an enrgizing circuit for an electromagnet coil,

(a) a control transistor having collector-emitter and base electrodes, the collector and emitter electrodes of which are included in a path for conducting current from a source to said coil,

(b) a current limiting resistor connected across said path,

(c) means responsive to the current flowing through said coil for reducing the conductivity of said transistor as the current through said coil increases,

((1) means connected to the emitter of said transistor in said path for preventing said transistor from being rendered nonconductive, and

(e) means connected to the collector of said transistor for preventing said transistor from saturating when it is in a high conductive state.

10. A circuit according to claim 9 in which said means for preventing said transistor from being rendered nonconductive and said means for preventing said transistor from being driven into saturation comprise strings of series connected diodes.

11. A driving circuit for an electromagnet coil including (a) a control transistor having base, emitter and collector electrodes,

(b) first diode means connected in series with the emitter electrode of said transistor,

(c) a path including said first diode means and the collector-emitter electrodes of said transistor for conducting current from a source to said coil,

(d) a current limiting resistor connected across said path,

(e) means for establishing a fixed bias potential on the base of said control transistor,

(f) means responsive to the current flowing through said coil for varying the potential on the emitter of said control transistor in accordance with the amount of said current, the conduction of said control transistor being reduced when said current through said coil is increased, said first diode means preventing said transistor from being driven to cutoif, and

(g) second diode means connected between the collector of said transistor and the source for preventing said transistor from saturating when said transistor is in a high conductive state.

12. A driving circuit for energizing an electromagnet coil in response to signals on an external line including (a) a pair of interconnected transistors,

(b) means connecting said coil and one of said transistors for providing a current path through said coil from a current source,

(0) means for applying said signals on said external line to the other of said transistors, and

((1) means for maintaining said transistors in either of two quiescent conducting states in accordance with the application of said signals on said external line, wherein the current flowing through said coil is sufficient to energize said coil when said transistors are in one of said quiescent conducting states and is insuflicient to energize said coil when said transistors are in the other of said quiescent conducting states,

(e) said external signals being applied in series aiding relationship with the current source to cause said transistors to assume said one quiescent conducting state when said external signals are present.

13. A driving circuit for energizing an electromagnet coil in response to signals on an external line including (a) a first transistor having at least base and emitter electrodes, (b) a second transistor for conducting current through said coil from a current source,

() means for applying said signal on said external line across the base and emitter electrodes of said one transistor, and

(d) means interconnecting said first and second transistors for causing current to flow through said second transistor and said coil to maintain a predetermined potential ditference between the base and emitter electrodes of said first transistor, said current being of one value in the absence of signals on said external line, wherein said one value of current flow through said coil is insufficient to energize said coil,

(e) said external signals being applied in series aiding relationship with the current source to cause a higher value of current to flow through said second transistor and said coil, said higher value of current being of suificient magnitude to energize said coil.

14. In a driving circuit for energizing an inductor in I response to signals on an external line,

(a) a first transistor,

(b) a second transistor for conducting current through said inductor from a current source,

(c) means interconnecting said first and second transistors to cause each of said transistors to be biased to a first conductive state by the other in the absence of signals on said external line causing a first amount of current to flow through said second transistor and said inductor, and

(d) means for applying said line signals to said first transistor and in series aiding relationship with the current source to cause said transistors to be driven to a second conductive state in which a higher amount of current than said first amount flows through said second transistor and said inductor, wherein said small amount of current is insuflicient to energize said inductor and said large amount of current is suflicient to energize said inductor.

15. A circuit for energizing the coil of an electromagnet in response to an external signal including (a) a first transistor having at least base and emitter electrodes,

(b) a second transistor for conducting current through said coil from a current source,

(c) means for applying said external signal across the base and emitter electrodes of said first transistor, and

((1) means interconnecting said first and second transistors for causing current to flow through said second transistor and said coil to maintain a predetermined potential difference between the base and emitter electrodes of said first transistor,

(e) said external signal being applied in series aiding relationship with the current source to cause said current through the second transistor to be of one value when no external signal is applied to said first transistor and be of a higher value than said one value when said external signal is applied to said first transistor, wherein said one value of current flowing through said coil is insufiicient to energize said coil and said higher value of current flowing through said coil is sufiicient to energize said coil.

16. A circuit for energizing the coil of an electromagnet in response to an external signal including (a) first and second transistors having first, second and base electrodes;

(b) means for connecting said coil between the first electrode of said second transistor and one side of a current source;

(c) means for applying said external signals across the second and base electrodes of said first transistor; ((1) first means for connecting the second electrode of said second transistor to the base of said first transistor;

(e) second means for connecting the first electrode of said first transistor to the base of said second transistor;

(f) means connecting the second electrode of said first transistor with the other side of the current source; (g) said first and second connecting means causing current of one value to flow through the path of the first and second electrodes of said second transistor and said coil when no external signals are applied to said first transistor, said current being of insuflicient magnitude to energize said electromagnet;

(b) said external signals being applied in series aiding relationship with the current source to cause increased current to flow through said, path including said first and second electrodes of said second transistor and said coil, said increased current being of sufiicient magnitude to energize said electromagnet.

References Cited by the Examiner UNITED STATES PATENTS 2,828,417 3/1958 Fleming et al. 307-885 2,967,991 1/ 1961 Deuitch 323-22 2,991,407 7/1961 Murphy.

3,026,454 3/1962 Goodwin 317-1485 X 3,075,128 l/1963 Cutsogeorge et al. 317-1485 3,084,310 4/1963 Schurr 317-1485 X 3,101,441 8/1963 Curry 323-22 3,109,980 11/1963 Wiley 307-885 3,116,441 12/1963 Gieifers 317-1485 3,123,746 3/1964 Wachowiak 317-1485 SAMUEL BERNSTEIN, Primary Examiner. LLOYD MCCOLLUM, Examiner. 

1. A CIRCUIT FOR ENERGIZING THE COIL OF AN ELECTROMAGNET IN RESPONSE TO AN EXTERNAL SIGNAL INCLUDING (A) A FIRST TRANSISTOR HAVING AT LEAST BASE AND EMITTER ELECTRODES, (B) A SECOND TRANSISTOR FOR CONDUCTING CURRENT THROUGH SAID COIL FROM A CURRENT SOURCE, (C) MEANS FOR APPLYING SAID SIGNAL ACROSS THE BASE AND EMITTER ELECTRODES OF SAID FIRST TRNASISTOR, (D) MEANS INTERCONNECTING SAID FIRST AND SECOND TRANSISTORS FOR CAUSING CURRENT TO FLOW THROUGH SAID SECOND TRANSISTOR AND SAID COIL TO MAINTAIN A PREDETERMINED POTENTIAL DIFFERENCE BETWEEN THE BASE AND EMITTER ELECTRODES OF SAID FIRST TRANSISTOR, (E) SAID EXTERNAL SIGNAL BEING APPLIED IN SERIES AIDING RELATIONSHIP WITH THE CURRENT SOURCE TO CAUSE SAID CURRENT THROUGH SAID SECOND TRANSISTOR TO BE OF ONE VALUE WHEN NO EXTERNAL SIGNAL IS APPLIED TO SAID FIRST TRANSISTOR AND TO BE OF A HIGHER VALUE WHEN SAID EXTERNAL SIGNAL IS APPLIED TO SAID FIRST TRANSISTOR, AND (F) MEANS FOR PREVENTING SAID SECOND TRANSISTOR FROM SATURATING.
 9. IN AN ENERGIZING CIRCUIT FOR AN ELECTROMAGNET COIL, (A) A CONTROL TRANSISTOR HAVING COLLECTOR-EMITTER AND BASE ELECTROODES, THE COLLECTOR AND EMITTER ELECTRODES OF WHICH ARE INCLUDED IN A PATH FOR CONDUCTING CURRENT FROM A SOURCE TO SAID COIL, (B) A CURRENT LIMITING RESISTOR CONNECTED ACROSS SAID PATH, (C) MEANS RESPONSIVE TO THE CURRENT FLOWING THROUGH SAID COIL FOR REDUCING THE CONDUCTIVITY OF SAID TRANSISTOR AS THE CURRENT THROUGH SAID COIL INCREASES, (D) MEANS CONNECTED TO THE EMITTER OF SAID TRANSISTOR IN SAID PATH FOR PREVENTING SAID TRANSISTOR FOR BEING RENDERED NONCONDUCTIVE, AND (E) MEANS CONNECTED TO THE COLLECTOR OF SAID TRANSISTOR FOR PREVENTING SAID TRANSISTOR FROM SATURATING WHEN IT IS IN A HIGH CONDUCTIVE STATE. 